1. Field of the Invention
The present invention relates to selective passivation of the channel walls of a channelled inkjet printhead component by the chemical vapour deposition of a passivant coating and, in another aspect, to a method of vacuum processing the surface of a component in general.
2. Description of Related Technology
The protection of a surface by deposition of a passivant layer (e.g. Silicon Nitride) by chemical vapour deposition in a vacuum is known in the art, for example from J. Applied Physics 66, No. 6, pages 2475-2480. The technique is predominantly used in the manufacture of semiconducting devices where restriction of coating to those areas where it is needed is achieved by use of photolithographic masking. As illustrated in FIG. 1(a), a layer of masking material 1 remains at selected locations on the substrate 2 following the dissolving of areas (indicated by 3) that have not been exposed to UV radiation in a preceding step. The entire substrate is then exposed to passivant coating as indicated at 4. FIG. 1(b) shows the substrate 2 following the coating process: passivant 5 has been deposited on the areas 3 whilst any passivant deposited on the masking material has been taken away with the removal of the masking material itself. The aforementioned masking process is known in the art and works well with the manufacture of devices on planar silicon wafers.
Passivation of channelled ink jet printheads is discussed in general terms in EP-A-0 364 136, incorporated herein by reference. FIG. 2(a) is a section through a printhead of the kind disclosed in this document taken perpendicular to the longitudinal axis of the channels: such devices comprise an array of channels 12 formed in a sheet 14 of piezoelectric material, suitably lead zirconium titanate (PZT), that has been poled in its thickness direction as indicated by arrows 15. Each channel is defined by side walls 16, a bottom surface 18 and a top sheet 20 and has formed on the surface of each side wall an electrode 34.
As is known, for example from EP-A-0 277 703 incorporated herein by reference, application of a electric field across electrodes 34 formed on opposite surfaces of a side wall 16 causes the piezoelectric material of the side wall to deflect in shear mode, thereby causing the ejection of an ink droplet from a nozzle associated with the channel.
As shown in FIG. 2(b), which is a sectional view taken along the longitudinal axis of a channel, such a nozzle 24 can be located at the forward end of each channel 12 which in turn comprises a forward part 36 of uniform depth which is plated to approximately one half the channel depth and a rearward part 38 of lesser depth which is fully plated over the base and walls to form connection tracks. The forward part of the electrodes in the channel apply the aforementioned actuating electric field whilst the rearward, connection track part of the electrodes are connected, e.g. by wire bonding, to actuating voltage supply means (not shown). Nickel, nichrome (an alloy of nickel and chromium) and aluminium have proved particularly suitable as electrode materials due to their high conductivity and suitability for wire bonding.
Subsequent passivation of the electrodes on the channel walls of such a printhead is necessary to protect the electrodes from attack by the ink contained in the channels during operation of the printhead. Aluminium in particular requires passivation to inhibit electrolysis and bubble formation or corrosion which could occur if the electrode were in direct contact with the ink. Protection is particularly desirable where the ink is aqueous or otherwise electrically conductive.
The composition of the passivation layer is chosen act as an electron and/or ion and/or ink barrier and is preferably configured so as to extend down one channel side wall, across the base of the channel, up the other channel side wall and over the top of that wall into the adjacent channel, thereby creating a continuous protective layer free of any edges under which ink might otherwise seep. A chemical vapour deposition process particularly suitable for the passivation of the "deep" channels shown in FIGS. 2(a) and (b)--i.e. channels having an aspect (height/width) ratio of at least 3:1--is disclosed in WO95/07820 incorporated herein by reference.
It will be appreciated that the connection track in the rearward part 38 of the printhead must be kept free of passivation in order that a connection (generally a wire bond) from the track to a driving circuit can be made. However, the aforementioned photolithographic masking techniques conventionally used when depositing a passivant layer by chemical vapour deposition have proved difficult to use in such a situation: in particular, application and removal of masking material on the walls and bottom surface of the rearward part of each channel (typically having a width of 60-90 .mu.m and a depth of 20-25 .mu.m) has proved complex and difficult. These problems are exacerbated where it is desired to mask not only the rearward part 38 but also a section of the full-depth, forward part 36 of the channel (typical width 60-90 .mu.m, typical depth 300-400 .mu.m), as proposed for the printhead constructions disclosed in co-pending International application no. PCT/GB97/01083(WO97/39897), belonging to the present applicant and incorporated herein by reference.